Visual Perception and Associated Disorders

Nasal retina crosses over at the optic chiasm.
Left visual field goes to right hemisphere.
Right visual field goes to left hemisphere.
Visual Pathway:
- Retina → Optic Nerve → Optic Chiasm → Optic Tract → Lateral Geniculate Nucleus (LGN) → Optic Radiation → Primary Visual Cortex (V1)

Measures the BOLD signal, which indicates the amount of blood flow and oxygen release to an activated brain area.
Principles of FMRI Retinotopy include:
- Polar Angle Mapping: Using movies of clockwise or counter-clockwise rotating wedges.
- Eccentricity Mapping: Using movies of expanding or contracting rings.

Polar Angle Mapping:
- Medial view of the left hemisphere shows:
  - Upper visual field maps to ventral V1.
  - Lower visual field maps to dorsal V1.
Eccentricity Mapping:
- Medial view of the left hemisphere shows:
  - Fovea maps to posterior V1.
  - Periphery maps to anterior V1.
  - Greater V1 representation for the fovea compared to the periphery.

Impairments in deriving meaning from visually presented stimuli, despite intact sensory and low-level vision, and normal language and semantic function.
Causes: strokes, tumors, anoxia, or neurodegeneration.

19th Century: Physicians reported cases of patients with brain damage who had intact sight but could not recognize objects.
Sigmund Freud: Introduced the term "agnosia" (from Greek for without knowledge) to describe these patients.

Apperceptive Visual Agnosia: Failure to process visual elements together for perception.
- Unable to identify, copy, or match drawings.
- Often arises from large and diffuse damage encompassing the posterior ventral stream.
Associative Visual Agnosia: Perception is stripped of meaning; the link is not made between stimulus and concept.
- Able to copy and match drawings but cannot identify them visually.
- Often arises from damage to ventral stream structures that are more anterior, including the anterior-most temporal lobes.
- Patients can copy pictures but have no idea what the pictures are; they know the concept and can recognize the same objects from the sounds they make.

Impaired in recognizing faces despite preserved visual acuity and minor visual field defects.
Often arises from bilateral or right hemispheric lesions encompassing the fusiform gyrus.

Impaired in recognizing landmarks or known scenes despite preserved visual acuity and minor visual field defects.
Often arises from bilateral or right hemispheric lesions encompassing the parahippocampal gyrus.

Impaired in shape and form discrimination despite preserved visual acuity and minor visual field defects.
Often arises from bilateral extra-striate lesions in the occipital and temporal cortex.

Developed visual form agnosia following carbon monoxide poisoning at 34 years old.
Extensively studied patient with visual form agnosia.
Considered the "Rosetta Stone" of Milner and Goodale’s two visual stream theory.
Example:
- When shown an object, DF could describe its material and color but not its form or function.
- "It’s made out of metal – is it aluminum? It’s got red plastic on it. Is it some sort of kitchen utensil?"
- "It's orange and has stripes. Is it a tiger?"

In a normal observer's brain, the LOC activates in response to intact stimuli compared to scrambled stimuli.
In DF's brain, there is an LOC lesion, resulting in impaired LOC activation.

Intact line drawings versus scrambled versions were presented for 4 seconds with a 12-second inter-stimulus interval.
Intact color photos versus scrambled versions were presented for 4 seconds with a 12-second inter-stimulus interval.

Comparison of intact vs. scrambled images in DF’s brain reveals:
- Intact line drawings minus scrambled versions show reduced activation in LOC due to the lesion.
- Intact color photos minus scrambled versions show reduced activation in the fusiform gyrus.